1. Field of the Invention
This invention relates to safety mechanisms for various power driven devices and, more particularly, to a universal load sensing gearbox therefor adapted at least to interrupt power thereto. The invention is especially but not exclusively useful for garage doors, gates, barriers and the like.
2. Description of the Prior Art
Safety mechanisms were introduced, for instance, to power driven closures, such as vertical sliding garage doors or pivoting barriers of the type found in parking lots, in order to avoid injury to people or damage to vehicles when these closures are lowered. The same applies for gates which slide horizontally to close for instance, a passageway, in a fence.
Various safety mechanisms have thus been developed, amongst which the most common are safety strips provided along the bottom edge of conventional upward acting overhead-type garage doors. Such safety strips consist of a movable bar which is displaced upon meeting an obstruction so as to trip a switch which is connected to the motor actuating the door.
In other cases, safety mechanisms have been embodied directly onto the shaft of the motor. For example, U.S. Pat. No. 3,633,313, issued to Lafontaine on Jan. 11, 1972, discloses a door operating device including a motor having a worm slidably mounted on its shaft and adapted to therewith. The worm is positioned on the shaft between a pair of bearings or bushings each carrying a limit finger adapted to trip a limit switch upon axial displacement of the worm and therefore of the bushings and limit fingers. The worm is held into its normal position by a pair of compressed springs. The worm meshes with a worm gear which is coupled in some way to a garage door for the raising and lowering thereof. When the garage door encounters an obstruction, the angular speed of the worm gear is reduced. Since the speed of the motor remains the same, the worm slides on the motor shaft and trips one of the limit switches to stop or reverse the motor.
U.S. Pat. No. 4,750,294 which issued on Jan. 14, 1988 also to Lafontaine, is directed to an electrically driven motor shaft and introduces a cage mounted on the motor drive shaft on each side of the worm to slide therewith. Compression springs which are positioned around a further shaft act on the cage and thus on the worm to maintain the latter in its normal central meshed engagement with the worm gear. The cage carries an actuator member which upon axial displacement of the cage along the shaft actuates the associated limit switch.
From the discussion of the above safety mechanisms, it appears that the safety strip is basically only suitable for vertical sliding garage doors. Furthermore, ice can freeze the safety strip thereby preventing the same from actuating the switch adapted to override the operation of the motor when an obstruction is encountered. Moreover, the safety strip and switch arrangement does not differentiate between the actuating thereof when the door is being lowered or when it is being raised. In the first instance, it is desirable that the motor not only stops operating but also reverses in case, for instance, a child is trapped underneath the door. On the other hand, the motor needs only to be stopped when the door is moving upwards.
Both the hereinabove presented Lafontaine patents offer a pair of distinct limit switches which are selectively tripped depending on the direction of rotation of the motor and thus on the direction of the door. Therefore, these switches can be wired differently to the motor in order to produce an appropriate effect thereon depending on the direction of displacement of the closure operated thereby.
However, the positioning of the safety mechanism, that is the worm, the cage structure and the springs, directly on the motor shaft results in a constant load thereon that causes the motor to be substantially rapidly destroyed. This is due to the fact that a motor shaft is not designed to sustain flexion forces.